The present invention relates to a new and improved converter-powered drive for an elevator and which is connected to an a.c.-voltage source.
In its more specific aspects, the present invention relates to a new and improved converter-powered drive for an elevator and connected to an a.c. voltage source. This drive contains a d.c.-electro-motor having an armature connected with the a.c. voltage source via a converter which has only one current-flow direction. The converter-powered drive further contains a switching circuit for torque reversal which is arranged between the converter and the armature and which comprises four thyristors. Two of the four thyristors are connected between the d.c.-negative pole of the converter and the armature and the other two of the four thyristors are connected between the d.c.-positive pole of the converter and the armature. There is further provided an extinguishing circuit containing at least one capacitor, an inductor, a resistor and a thyristor. The extinguishing circuit protects the thyristors of the converter upon the occurrence of excess current and dissipates, by means of the resistor, the energy appearing in the armature circuit.
In a similar drive as known, for example, from U.S. Pat. No. 4,209,082, granted June 24, 1980, the capacitor of the extinguishing circuit is charged each time before a travel start of the elevator car conjointly with the ignition of the thyristors in the converter. The extinguishing circuit possesses a detector which causes the ignition of the thyristor in the extinguishing circuit upon the occurrence of a line voltage or power failure and, as a result, the thyristors of the converter are cut-off by the discharge of the capacitor.
In the aforementioned converter-powered drive, it is possible that, for example, due to a defective component, it is unnoticed by the control that the capacitor is not charged so that the thyristors cannot be cut-off. It is a further disadvantage that the extinguishing circuit cannot be activated upon the occurrence of an excess current which is not caused by a line voltage or power failure. The switching circuit for torque reversal must be controlled at the occurrence of a line voltage or power failure as well as during normal operation in such a manner that the energy can be dissipated or conducted away and which energy is generated by the motor when the motor is operated in the manner of a generator. No solution for this problem is given in the aforementioned U.S. Pat. No. 4,209,082. Additionally, it is possible that the energy which is generated during the generator operation of the motor, is only incompletely dissipated or conducted away when a main contactor is too rapidly opened and which contactor is arranged between the extinguishing circuit and the armature.
In drives of the aforementioned type, the converter is designed in most cases as a three-phase a.c.-bridge circuit which generates a six-pulse output voltage. In order to reduce the unfavorable effects on the armature circuit due to the d.c.-ripple and also the interfering effects of harmonics on the mains supply occurring during the operation of such a circuit, there can be employed, for example, filter circuits. Thus, a low-pass filter is arranged between the converter and the armature of the drive motor in the converter-powered drive as known, for example, from U.S. Pat. No. 3,821,628, granted June 28, 1974. This drive comprises a converter constructed from two three-phase a.c.-bridge circuits which are connected in parallel opposition to each other. The machine noise caused by the harmonics is intended to be attenuated by the low-pass filter. In order to also maintain small the distortion on the mains, additional filters would have to be employed, whereby the equipment costs would be further increased.
It is otherwise known to reduce the d.c.-ripple by using converters with higher pulse numbers. There is thus described, for example, in German Patent No. 1,107,784, granted May 31, 1961, a twelve-pulse converter for reversable drives in which the parallel opposition connection is constructed with interphase reactors. In this converter two transformers with six-phase secondary windings which are offset from each other by 30.degree., are used in a star or Y-connection for the power supply. The number of rectifiers in this converter corresponds to twice the pulse number and thereby the costs are considerably increased. Considering further the expense due to the interphase reactors, such a converter would appear unsuited for use in combination with elevators.